1. Field of the Invention
This invention relates to a supported metallocene-alumoxane catalyst and a process for preparing this catalyst for use in polymerization of olefins and especially in the gas phase or liquid phase polymerization of olefins. The invention particularly relates to the substitution of triisobutylaluminum for trimethylaluminum in the production of those metallocene-alumoxane catalysts which are supported on silica gel containing from about 6 to about 20 percent by weight adsorbed water. The resulting material is dried to a free-flowing powder to yield a supported catalyst which is active for the homo or copolymerization of polymerizable-olefins.
2. Background to the Invention
Olefin polymerization catalysts comprising a metallocene and an aluminum alkyl component were first proposed in about 1956. Australian patent No. 220436 proposed for use as a polymerization catalyst a bis-(cyclopentadienyl) titanium, zirconium, or vanadium salt as reacted with a variety of halogenated or unhalogenated aluminum alkyl compounds. Although capable of catalyzing the polymerization of ethylene, such catalytic complexes, especially those made by reaction with a trialkyl aluminum, had an insufficient level of catalytic activity to be employed commercially for production of polyethylene or copolymers of ethylene.
Later it was found that certain metallocenes such as bis-(cyclopentadienyl) titanium, or zirconium dialkyls in combination with aluminum alkyl/water cocatalyst formed catalyst systems for the polymerization of ethylene. Such catalysts are discussed in German Patent Application No. 2,608,863 which discloses a polymerization catalyst for ethylene consisting of bis-(cyclopentadienyl) titanium dialkyl, trialkyl aluminum and water. German Patent Application No. 2,608,933 discloses an ethylene polymerization catalyst consisting of a cyclopentadienyl zirconium salt, a trialkyl aluminum cocatalyst and water. European Patent Application No. 0035242 discloses a process for preparing ethylene and atactic propylene polymers in the presence of a cyclopentadienyl transition metal salt and an alumoxane. Such catalysts have sufficient activity to be commercially useful and enable the control of polyolefin molecular weight by means other than hydrogen addition--such as by controlling the reaction temperature or by controlling the amount of cocatalyst alumoxane as such or as produced by the reaction of water with an aluminum alkyl.
To realize the benefits of such catalyst systems, one must use or produce the required alumoxane cocatalyst component. An alumoxane is produced by the reaction of an aluminum alkyl with water. The reaction of an aluminum alkyl with water is very rapid and highly exothermic. Because of the extreme violence of the reaction the alumoxane cocatalyst component has, heretofore, been separately prepared by one of two general methods. Alumoxanes may be prepared by adding an extremely finely divided water, such as in the form of a humid solvent, to a solution of aluminum alkyl in toluene or other aromatic hydrocarbons. The production of an alumoxane by such procedure requires use of explosion-proof equipment and very close control of the reaction conditions in order to reduce potential fire and explosion hazards. Also, fine solid particles are generated in the head space of the reactor which can plug the vent and the transfer tube and cause the shut down of the process. For this reason, it has been preferred to produce alumoxane by reacting an aluminum alkyl with a hydrated salt, such as hydrated copper sulfate. In such procedure a slurry of finely divided copper sulfate pentahydrate and toluene is formed and mantled under an inert gas. Aluminum alkyl is then slowly added to the slurry with stirring and the reaction mixture is maintained at room temperature for 24 to 48 hours during which a slow hydrolysis occurs by which alumoxane is produced. Although the production of alumoxane by a hydrated salt method significantly reduces the explosion, fire hazard, and fine generation inherent in the wet solvent production method, production of an alumoxane by reaction with a hydrated salt must be carried out as a process separate from that of producing the metallocene-alumoxane catalyst itself, is slow, and produces hazardous wastes that create disposal problems. Further, before the alumoxane can be used for the production of an active catalyst complex the hydrated salt reagent must be separated from the alumoxane to prevent it from becoming entrained in the catalyst complex and thus contaminating any polymer produced therewith.
U.S. Pat. No. 4,431,788 discloses a process for producing a starch filled polyolefin composition wherein a trialkyl aluminum is first reacted with starch particles. The starch particles are then treated with a (cyclopentadienyl)-chromium, titanium, vanadium or zirconium alkyl to form a metallocene-alumoxane catalyst complex on the surface of the starch particles. An olefin is then polymerized about the starch particles by solution or suspension polymerization procedures to form a free-flowing composition of polyolefin-coated starch particles. German Patent No. 3,240,382 likewise discloses a method for producing a filled polyolefin composition which utilizes the water content of an inorganic filler material to directly react with a trialkyl aluminum and produce thereon an active metallocene alumoxane catalyst complex. Polymer is produced by solution or gas phase procedures at the filler surface to uniformly coat the filler particles and provide a filled polymer composition.
German Patent No. 3,240,382 notes that the activity of a metallocene-alumoxane catalyst is greatly impaired or lost when prepared as a surface coating on an inorganic material. Although German Patent No. 3,240,382 suggests that an inorganic material containing absorbed or adsorbed water may be used as a filler material from which the alumoxane cocatalyst component may be prepared by direct reaction with a trialkyl aluminum, the only water containing inorganic filler materials which are identified as capable of producing the alumoxane without adversely affecting the activity of the metallocene alumoxane catalyst complex are certain inorganic materials containing water of crystallization or bound water, such as gypsum or mica. German Patent No. 3,240,382 does not illustrate the production of a catalyst coated inorganic filler material wherein the inorganic material is one having absorbed or adsorbed water. Nor does German Patent No. 3,240,382 describe an inorganic filler material having absorbed or adsorbed water which has surface area or pore volume properties suitable for service as a catalyst support for a gas phase polymerization procedure. All these publications also teach that only methylalumoxane formed by reacting trimethylaluminum with water, has high enough activity for polyolefin polymerization. Other trialkyl aluminum compounds do not form high activity alumoxane when react with water.
My copending U.S. patent application Ser. No. 134,413, now U.S. Pat. No. 4,912,075 discloses a method by which the requisite alumoxane cocatalyst component for a supported metallocene gas phase polymerization catalyst may be safely and economically prepared by addition of an "undehydrated" silica gel to a trialkyl aluminum solution. My co-pending application illustrates the production of highly active silica gel supported metallocene-alumoxane catalyst wherein trimethylaluminum is utilized to form the alumoxane. Although the reaction product of triethylaluminum with water is known to form an ineffective cocatalyst, a highly active catalyst system is formed in accordance with the method disclosed in my co-pending application Ser. No. 268,834, now U.S. Pat. No. 4,925,821 by reacting triethylaluminum with undehydrated silica gel followed by reacting with metallocene.
My copending U.S. application Ser. No. 263,572 teaches the use of a mixture of TEAL and TMA to produce the alumoxane component of a metallocene-alumoxane catalyst in a less expensive process than the use of TMA alone. The use of this TEAL/TMA mixture produces an alumoxane which, in combination with a metallocene, provides a catalyst more active for olefin polymerization than those metallocene-alumoxane catalysts using only TEAL. Furthermore, it has been found that the TEAL/TMA mixture-based metallocene-alumoxane catalyst reduces the amount of solid waste particles that accumulate in the head space of reactors thereby eliminating or reducing vent-line plugging and costly reactor down time. The TEAL in the mixture also tends to reduce the violence of the reaction produced when TMA alone is contacted with water to produce an alumoxane component. Thus, the TEAL/TMA based catalysts provide high activity, lower cost catalysts which are safer to produce and which offer reduced operating costs due to lower reactor down times.
Despite the activity and cost improvements already achieved in processes for polymerizing olefins using metallocene-mixed alumoxane catalysts, it is yet desirable to produce even higher activity catalysts.